A safeword is a codeword that is sometimes used in BDSM to mean stop.
This is generally used so that the bottom can scream "no, stop", etc. as much as s/he wants without really meaning it, and still have a way of indicating a serious desire that the scene stops. Accordingly, a safeword is usually a word that the person would not ordinarily say during sex, such as red, tree, anomaly, or even safeword.
Some partners have different gradations of safeword, such as yellow to mean "that hurts" or "stop doing that" without stopping the scene, and red to mean "let's stop the scene". In other circumstances the safeword may not be a "word" at all, but a signal like dropping a ball or similar, or making three clear and rhythmic grunts or vigorous motions in the case of a person who is gagged or bound.
In the case of sexual role-play, some simply drop out of character to safeword, such as having the submissive address the dominant by his/her first name.
While many in the BDSM community consider safewords an essential part of safe play, there is a significant contingent that does not have any such term in their relationships or their play, relying instead on the top to monitor the condition of the bottom and stop if necessary. In such circumstances the bottom or submissive must have consented not to have control over the duration of the scene in advance.
Interestingly some of those who recommend safewords do not, themselves, use them though this is not often discussed in public. There is an undercurrent assumption that play without safewords is an "advanced" technique and should not be advocated in the hearing of novices.
Warning: Note that BDSM activity without a safeword is regarded by many as inadvisable and dangerous.
